With the announcement by the Federal Communications Commission in 1967 of its intention to grant permission for telephone company customers to install and maintain their own terminal equipment, there arose a concern on the part of telephone companies that this will increase the possibility that voltages hazardous to telephone craftspersons will be accidentally applied by the customer to the telephone line. Hazardous voltages are introduced at the site of terminal equipment from two major sources. The first of these sources is crosses with the phase or hot conductor of a 110-volt commercial power line, and the second is crosses associated with key telephone system ringing generators.
By far the most common situation that can be expected is that such crosses will result in a voltage being applied to one or both tip and ring line conductors with respect to ground. Such a voltage is referred to as a longitudinal voltage, and it produces an unbalanced current flow through the telephone line when a path to ground is provided. It is expected that crosses that result in a voltage being applied only between the tip and the ring line conductors, referred to as a metallic voltage, will be less common. Such a voltage produces a balanced equal and opposite current flow through the tip and ring line conductors when a path between the conductors is provided.